Add incremental tests

tests/cases/fourslash/incrementalParsingDynamicImport1.ts

@@ -0,0 +1,17 @@
+/// <reference path="fourslash.ts"/>
+
+// @lib: es6
+
+// @Filename: ./foo.ts
+//// export function bar() { return 1; }
+
+//// var x1 = import("./foo");
+//// x1.then(foo => {
+////    var s: string = foo.bar();    
+//// })
+//// /*1*/
+
+verify.numberOfErrorsInCurrentFile(1);
+goTo.marker("1");
+edit.insert("  ");
+verify.numberOfErrorsInCurrentFile(1);